1. Field of the Invention
This invention relates to a thin film semiconductor device and a manufacturing method thereof. More specifically, it relates to a constitution of a substrate for integrating and forming a thin film transistor thereon.
2. Description of the Related Art
Generally, in a thin film semiconductor device, a thin film transistor is integrated and formed on an insulation substrate, for example, made of glass. The thin film transistor has to be formed through steps, such as CVD (Chemical Vapor Deposition), cleaning and heat treatment. Since different steps are successively applied, the substrate has to be transported between the stages for each of the processes. If the substrate is deformed, such as by warping, automatic transportation using a manipulator is not possible. Therefore, a substrate for use in the thin film semiconductor device requires heat resistance capable of enduring the temperature for forming the thin film transistor. Further, it is necessary for a thickness of a predetermined level or more, so as not to cause warping deformation during transportation.
On the other hand, products using the thin film semiconductor device as a component can include, for example, an active matrix type liquid crystal display. When a liquid crystal display is used for portable equipment, the thin film semiconductor device is required for a structure reduced in weight and of less fracture. Therefore, manufacturing conditions required for the substrate and conditions for the products do not often agree with each other, and there is a market complaint against the products. As described above, use of glass for the substrate is suitable in view of the manufacturing conditions such as heat resistance. However, in view of the product conditions, the glass substrate has a drawbacks of heavy weight and easy cracking. For the liquid display used in portable electronic equipment, such as palm-top computers or portable telephones, it is desirable for the material to have a cost that is as low as possible, to be reduced in the weight, to endure some deformation and to crack less upon dropping. Actually, the glass substrate is heavy, sensitive to deformation and likely to be broken by dropping. That is, there is a difference between restrictions due to manufacturing conditions and preferred characteristics required for the products, and it is difficult to satisfy both of the conditions or characteristics, and this is left still unresolved.
Various countermeasures have been proposed so far in order to overcome such problems. For example, it has been attempted to use a plastic substrate, for example, by lowering the process temperature for the thin film transistor as much as possible. However, the plastic substrate suffers from a larger deformation compared with the glass substrate, and no satisfactory products have been obtained at present (N. D. Young, et al., Euro Display '96 Digest, 555, 1996). Further, there has been proposed a countermeasure of once forming a thin film transistor to a provisional substrate and then transferring the same to another substrate which is disclosed, for example, in Japanese Published Unexamined Patent Application Hei 11-243209. However, this method is complicated in the transfer step and involves a problem in view of the productivity. Further, since the provisional substrate used for forming the thin film transistor and a substrate for mounting the thin film transistor as a product are different, characteristic are liable to fluctuate in the thin film transistor in view of the problems, such as stresses and the like.